1. Field of the Invention
This invention relates to imaging systems. More specifically, this invention relates to apparatus used to detect electromagnetic radiation irradiating such systems.
While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
Forward Looking Infrared (FLIR) thermal imaging systems are generally used to view scenes by using the infrared energy emitted by the scene. FLIR systems commonly include a telescope, imaging optics and a dewar. The dewar contains a detector which emits a signal in response to infrared energy emitted by the scene. Typically, the detector is cooled to less than one hundred degrees Kelvin to reduce internal thermal noise and thereby improve sensitivity. A cooled coldshield is also typically installed within the dewar. This coldshield is configured so -as to not vignette (obscure) radiation focused on the detector by the telescope and imaging optics. Further, the coldshield enhances system performance by minimizing the amount of radiation striking the detector from sources other than the scene.
The performance of FLIR systems may be degraded by several optical phenomena. Included among these potential difficulties are ghosting (a form of stray light) and narcissism. Ghosting occurs when radiation either inside or ouside of the instantaneous field of view (IFOV) is partially reflected off (typically two) surfaces and thereby laterally displaced from the original radiation path. This errant radiation then strikes the detector at a location different from that at which it would had the radiation not been laterally displaced. Hence, the detector sees multiple images of the radiation source.
Narcissus, as implied by the term, occurs when the detector sees an image of the cold portion of itself (or of cold structures within the dewar) superimposed on the image of the scene. For this xe2x80x9ccold imagexe2x80x9d to be detrimental to system performance it must be time-varying, as is the case in scanned systems. In addition, the narcissus generating partially reflecting surface(s) must lie beyond the scanner from the detector. Unfortunately, the detector sees a very cold narcissistic image when the scanner mirror is xe2x80x9con axisxe2x80x9d and looking back into the cold dewar, but sees a relatively constant warm image when the scanner mirror moves slightly off axis. This generates a xe2x80x9ccold spikexe2x80x9d background as the scanner mirror moves through the xe2x80x9con axisxe2x80x9d position. Moreover, the cold spike is generally located in the center of the resultant imagexe2x80x94typically that part of the scene in which the viewer is most interested.
At least two characteristics of a given FLIR system directly impact the severity of potential narcissus problems. The first pertains to the reflectance of the narcissistic surface(s). A higher reflectance will result, proportionally, in a colder spike. Second, the curvature of the reflecting surface defines the degree of focus of the narcissistic image. If the curvature is such that the surface is normal to all incident rays from the detector, then the detector will see a sharply focused image of itself.
Several schemes have been employed to mitigate narcissism. In one such scheme the system is designed so as to minimize xe2x80x9cdetector to detectorxe2x80x9d imagery. That is, optical surfaces within the system are adjusted so that reflections onto the detector are xe2x80x9cdefocused.xe2x80x9d Unfortunately, this correction of narcissism by design, although favorable, is sometimes limited in efficacy by other constraints.
A second partial remedy for narcissism is known to those skilled in the art as ARC-NARC (Automatic Responsivity Correction Narcissus). This approach involves the superposition of an image of a warm source over the narcissistic image on the detector. At the point in time when the scanning mirror is positioned such that the detector xe2x80x9cseesxe2x80x9d itself most fully, the detector will also see a superimposed image of the warm source. By adjusting the temperature of the warm source, the narcissistic cold spike in the resultant scene image is ostensibly masked by the image of the warm source. However, difficulties in xe2x80x9cmatchingxe2x80x9d the (blackbody) radiation curves associated with the warm and cold sources to generate a resultant radiation profile indistinguishable from that of the ambient environment have limited the efficacy of this technique.
A third technique employed for reducing narcissism involves filtering to a relatively narrow spectral band radiation from the scene that is seen by the detector. This technique, however, cannot be expected to remedy narcissism to the extent desired in certain FLIR applications.
In a fourth method of narcissus reduction, a xe2x80x9cdetector mirrorxe2x80x9d is placed within the dewar immediately in front of the detector. The mirror has an aperture so as to not vignette the field of view of the detector. The center of curvature of the mirror is typically located on the optical axis at the center of the coldshield aperture. Any ray passing through the aperture and striking the detector mirror is reflected back out the aperture at a conjugate height from the optical axis. Thus, if one were looking into the dewar of this configuration, the detector would be the only cold appearing object. The detector mirror would appear warm because the viewer would be looking back at an inversed image of the viewer (warm). The coldshield outer surface is typically gold plated for thermal considerations, so the viewer would see the warm xe2x80x9coutsidexe2x80x9d world in reflection off this surface. That leaves only the detector itself to appear cold. The detector mirror in effect optically transforms certain xe2x80x9cphysically coldxe2x80x9d objects within the field of view of the detector into appearing warm. The detector mirror provides the added benefit of reducing the thermal load on the dewar by reflecting radiation which would normally be absorbed thereby. Although this approach may result in a reduction in narcissism, the efficacy of this approach is limited when the detector image is sharply focused onto the detector.
Further, although detector mirrors reduce narcissism while leaving most other aspects of optical performance essentially unchanged, detector mirrors tend to contribute to ghosting. In particular, radiation entering the aperture in the coldshield which is not focused on the detector (i.e. stray light) may be reflected by the detector mirror, This reflected stray light may again be reflected by other surfaces onto the detector, thus generating image ghosts. It follows that the addition of the detector mirror may increase the susceptibility of the system to stray light ghosting.
Hence a need in the art exists for an infrared detection apparatus having decreased sensitivity both to narcissism and to stray light ghosting.
The need in the art for an infrared detection apparatus having decreased sensitivity both to narcissism and to stray light ghosting is addressed by the improved detector assembly of the present invention. The improved detector assembly of the present invention includes a housing having an input aperture (coldshield) in communication with a chamber within the housing. A detector for sensing electromagnetic energy passing through the input aperture within a first field of view is mounted within the chamber. Also mounted within the chamber is a first mirror for reflecting energy passing through the input aperture within a second field of view outside of the first field of view. The improved assembly of the present invention further includes a second mirror mounted within the chamber for reflecting energy reflected by the first mirror through the input aperture.